How to control the circulating water flow rate in a circulating water cooler
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As the title suggests, for coolers that use circulating water as a cooling medium, the circulating water is directly fed into the cooler, and there are no control valves or shut-off valves to manually regulate the flow rate. So how can it be ensured that the flow rate of the circulating water as well as the inlet and outlet temperatures are in line with those calculated by the software? For example, according to the calculations, 10 t/h of circulating water is required, with a supply pipe pressure of 0.5 Mpa and a return pipe pressure of 0.25 Mpa. The inlet and outlet temperatures are calculated to increase from 33°C to 43°C, and the pressure loss is estimated to be 30 kpa. The circulating water flows directly from the main pipe back to the hot water main pipe, without any valves for regulating flow rate. So how can we ensure that the circulating water flow rate remains at 10 t/h and the pressure loss stays at 30 kpa? Why doesn’t it flow at the maximum possible pressure loss, for instance at 100 t/h, resulting in a pressure loss of 0.25 Mpa and a temperature difference of only 1°C? In that case too, flow would be possible. This is the same as calculating the maximum flow rate of a pipeline; we both use the economic flow velocity for such calculations. But why does the flow follow this economic flow velocity? Why isn’t it restricted only by physical laws? For example, if the pressure difference is 0.25 MPa, then the flow rate should be high enough that the pressure loss reaches 0.25 MPa, after which it can no longer increase, right? Similarly, may I ask how to calculate the maximum flow rate of a pipeline? https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5668444&page=1#pid26880545
In a cooling water system without control valves, ensuring that the flow rate and pressure meet the preset parameters relies primarily on the system design and the characteristics of the components themselves. Specifically: 1. **Pipe size design**: The flow rate is influenced not only by the pressure difference but also by factors such as the diameter and length of the pipe, as well as the friction coefficient of its inner wall. During design, the appropriate pipe diameter is determined through calculations to ensure that the required flow rate can be achieved under the designed pressure difference. If the pipe diameter is chosen properly, even in the absence of a pressure regulating valve, the flow rate will be confined to a certain range due to physical factors such as pressure loss and pipe resistance. 2. **Pump selection**: The water pump in the system is also a critical component; its flow rate and head must be compatible with the system design. The operating point of the pump should be set near its most efficient point, while meeting the system’s flow and pressure requirements. 3. **Local resistance calculation**: During design, it is necessary to calculate the local resistance (such as that associated with elbows, valves, filters, etc.) and frictional resistance throughout the circulation system, as these factors directly affect the pressure loss in the flow. 4. **Thermodynamic and hydrodynamic calculations**: Use relevant software or specialized formulas to calculate the flow rate of water and its state changes under specific temperature and pressure conditions. As for why abnormal operating conditions in which the flow rate is much higher than the designed value and only a small temperature difference occurs, this is mainly because the designed system usually has a certain safety margin; moreover, an excessively high flow rate in actual operation can cause other system components such as pumps and pipes to become overloaded, thereby affecting the stability and efficiency of the entire system. Regarding the **calculation of the maximum flow rate for pipes**, the following factors are usually taken into account: - **Pipe diameter and length**** ； - **Density and viscosity of fluids** ； - **Maximum allowable pressure loss** ； - **Maximum allowable flow rate** (generally affected by factors such as corrosion or wear). Calculations are usually performed using the Bernoulli equation in combination with the continuity equation for fluids. In practical applications, it is also necessary to take into account the actual operating conditions and safety factors; typically, some empirical formulas or specialized software are used for detailed calculations. .
The presence of a pump is explainable, as the flow rate of the pump limits the flow rate entering the heat exchanger. But in many cases, the cooler is connected directly to the main circulating water pipe, rather than through a pump. If the pressure in the return main line during circulation is 0.5 MPa and the return water pressure is 0.25 MPa, then why does the flow rate of the circulating water follow the predetermined pressure drop (such as 30 kPa) and the predetermined temperature difference (such as 32°C at the inlet and 42°C at the outlet, resulting in a temperature difference of 10°C)? Why doesn’t the flow rate become very high, with a pressure difference of only 0.25 MPa and a temperature difference of just 1°C?
Thank you for the reply. I carefully read your response, and I understand that it is mainly divided into two parts: one is the pump, and the other is the pressure loss (including pipe pressure loss, fluid viscosity resistance, and things of that kind). It makes sense to have a pump; the flow rate delivered by the pump is the flow rate through the heat exchanger. But there is another scenario in which the circulating water comes directly from the main circulating water pipe, rather than passing through a pump. Then why doesn’t this scenario lead to the abnormal operating condition you described – one in which the flow rate is much higher than the designed value, resulting in only a very small temperature difference? What you mentioned – \"system overload, fluid viscosity, pressure loss\" – can all be summarized as pressure loss in my view. Why is the pressure in the return pipe 0.25 MPa? Yet after passing through the heat exchanger, the pressure loss decreases by only 30 kPa before the fluid returns; why doesn’t the flow rate increase further, causing the pressure loss to rise to 0.25 MPa, which is just enough for the fluid to return? What other factors besides pressure loss limit the flow rate?
Who told you that there are no valves for control? Any properly designed heat exchanger must have valves
However, the circulating water coolers that I often see on site do not have valves to control the flow rate. In many cases, the circulating water flows through the tube side, with only gate valves installed at the inlet and outlet of the equipment; these valves are always left fully open and are used solely to isolate the equipment, not to regulate the flow rate.
What I saw is completely different from what you described; most of them come with control valves or stop valves. Those that don’t require any adjustment are just used casually, without regard to their quality
Everyone overlooks the heat transfer area of the condenser. No matter how large the pressure difference is, a small heat transfer area simply cannot achieve the desired effect
1. Adjusting the opening degree of the manual valve also has a certain effect on reducing pressure drop. 2. The economic flow rate already takes into account a portion of the pressure loss; under normal circumstances, it can approximate the relationship among dynamic head, static head, and pressure loss as described by the Bernoulli equation. 3. The actual flow rate of the circulating water is not necessarily exactly equal to the flow rate at the optimal velocity; the actual heat exchange efficiency is influenced by various factors. Within a certain range, an increase in the circulation water flow rate has a minor impact on the heat exchange efficiency, and this factor is often ignored in applications where heat exchange is not of great importance. Adjustment valves are installed at all key heat exchange control points for regulation.